Any fort interested in holding out in a siege situation needs its own source of drinkable water, which in Fort Knox's case took the form of a cistern. In places where one could not just dig straight down and make a traditional well, rainwater runoff was collected in a cistern, which was a large sealed underground recepticle. This water, while perhaps not absolutely delicious, would serve the purpose of keeping dudes hydrated in a pinch. A cistern did not, however, look like this. Maybe a particularly well-regarded child fell into Fort Knox's cistern, which was then completely blocked with bricks and granite...since that's all that can be seen of the cistern today.